The high affinity IgE receptor belongs to a class of receptor which lacks intrinsic enzymatic activity but activates non-receptor tyrosine and serine/threonine and phosphatases. This activations leads to phosphorylation and activation of phophatidylinositol-specific phospholipase C, an increase in intracellular calcium and ultimately the cell degranulation and the release of the mediators of allergic reactions. Last year we had identified a distinct role for the beta and gamma subunit: beta but not gamma associates with the tyrosine kinase lyn in resting conditions, and gamma but not beta can activate the tyrosine kinase syk. Based on our findings, we had proposed a model of Fc(epsilon)RI activation in which, upon receptor aggregation lyn phosphorylates both beta and gamma, and the phosphorylation of gamma is necessary to recruit syk and activate syk. We now have tested this model of activation by reconstitution the various elements in fibroblasts. This is accomplished by infection of the fibroblasts expressing the tetrameric Fc(epsilon)RI receptor with recombinant vaccinia viruses. In this system, Lyn is the only kinase required for receptor phosphorylation. Cotransfection of Syk does not alter the phosphorylation state of Fc(epsilon)RI due to Lyn alone. The presence of the tetrameric receptor and Lyn is also required for the formation of a receptor-Lyn-Syk complex, which is increased after receptor aggregation. Thus, the cooperation between the two kinases and the beta and gamma subunits is necessary for the full capacity of the Fc(epsilon)RI receptor.